Rechargeable energy storage systems having a large number of batteries that can charge and discharge with great efficiency may become increasingly important in the coming years. For instance electric and hybrid cars may benefit from larger energy storage systems and efficient charging and discharging. To supplement wind and solar power generation when there are lulls in the wind or diminished sunlight, large and efficient energy storage systems will be needed in homes and as part of the power grid. Even today's rechargeable laptop computers and cell phones can benefit from more efficient charging and recharging. All of these energy storage systems can operate more effectively if the voltage between batteries is balanced. Current systems for balancing voltages are inefficient and these inefficiencies typically only get worse as the number of batteries increases (larger energy storage systems).
A battery is a combination of one or more electrochemical cells connected in series. Each cell produces a voltage, and when connected in series, a stack of cells (battery) produces a voltage equal to the sum of the cells comprising the battery. Each battery thus produces a voltage, and when connected in series, an energy storage system (comprising one or more batteries connected in series) produces a voltage equal to the sum of the batteries comprising the energy storage system. Voltage imbalances between batteries decrease system efficiency and limit the number of batteries that can be connected in series. Importantly, voltage imbalances may become harder to mitigate as the number of batteries connected in series increases.
At the same time, the number and importance of applications requiring large rechargeable energy storage systems are growing. For instance, homes relying to a large extent or exclusively on wind and/or solar power require large energy storage systems in order to supplement the wind and/or solar power when there is low wind speed or a lack of sunlight. The same problem threatens future power grids since they too will rely largely on wind and solar power. Thus, large energy storage systems are a key to enabling renewable energy to power homes and power grids. Many other applications will also benefit from or require large rechargeable energy storage systems.